Voltage-controlled electronic power switches comprise two load electrodes—usually termed “source” and “drain” for a MOSFET, “emitter” and “collector” for an IGBT—between which the load current to be gated flows through a load path of the semiconductor switch (in other words the drain-source current path) as well as a gating electrode—usually termed “gate”—whose difference in potential to that of the load electrode, usually termed “source”, is decisive as to whether the load path of the semiconductor switch is a high-impedance (OFF state of the switch) or low-impedance (ON state of the switch) or is in some state in between. When the reference potential of the gate electrode (in other words the potential of the source electrode) is floating, i.e., may vary referenced to the constant reference potential “GND” depending on the operating mode of the circuit, then the voltage supply of the driver circuit activating the gate electrode needs to be operated at this floating reference potential, as is the case, e.g., with high-side switches, i.e., switches circuited between an upper operating potential and the load.
In a half-bridge inverter consisting of the same type of voltage controlled electronic switches, two of these power switches are circuited in series between the poles of an operating voltage so that the source electrode of the first high-side power switch is connected to the drain electrode of the second low-side power switch in a common circuit node, the potential of which oscillates in time between the upper and lower limit of the operating voltage. This potential is the floating reference potential for the driver circuit gating the first (upper) power switch. The supply voltage of the driver circuit as the source for gating the first power switch needs to be correspondingly referenced also to the potential of this “floating” circuit node.
For this “floating” power supply of the driver circuits for bridge circuits in the voltage range above a 100 V intermediate circuit voltage and above a power of 0.5 kW use is made of either a bootstrap circuit or an isolated power supply, such as, e.g., switched power supplies or DC/DC converters.
In the bootstrap circuit a diode and capacitance are circuited in series with the load current branch of the power switch. When the power switch is OFF, the capacitance is charged, its voltage being tapped as the supply voltage for the driver circuit. The limits of the bootstrap circuit are dictated by a power switch when operated ON for a relatively long time. Since, during this time, the capacitance connected in parallel is only discharged, it is just a question of time until it is no longer able to satisfy its function of powering the driver circuit.
Isolated power supplies, for example transformers or isolated switched power supplies, are relatively complicated and expensive to achieve.
There is thus a need to provide a power supply circuit for driver circuits gating power semiconductor switches which is uncomplicated, achievable especially without having to use transformers and inductances and which can be integrated together with power semiconductors and drivers into an intelligent power module (IPM).